Intervalley scattering in hexagonal boron nitride

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Intervalley scattering in hexagonal boron nitride

G. Cassabois, P. Valvin, and B. Gil

Phys. Rev. B 93, 035207 – Published 28 January 2016

Abstract

We report photoluminescence experiments bringing the evidence for intervalley scattering in bulk hexagonal boron nitride. From a quantitative analysis of the defect-related emission band, we demonstrate that transverse optical phonons at the $K$ point of the Brillouin zone assist inter- $K$ valley scattering, which becomes observable because stacking faults in bulk hexagonal boron nitride provide a density of final electronic states. Time-resolved experiments highlight the different recombination dynamics of the phonon replicas implying either virtual excitonic states or real electronic states in the structural defects.

Received 11 December 2015

DOI:https://doi.org/10.1103/PhysRevB.93.035207

Physics Subject Headings (PhySH)

Band gap Phonons

Nitrides Semiconductor compounds

Photoluminescence

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

G. Cassabois, P. Valvin, and B. Gil^*

Laboratoire Charles Coulomb (L2C), UMR 5221 CNRS-Université de Montpellier, F-34095, Montpellier, France

^*bernard.gil@umontpellier.fr

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Vol. 93, Iss. 3 — 15 January 2016

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Images

Figure 1
(a) Normalized photoluminescence (PL) signal intensity in bulk hBN at 10 K, for an excitation at 6.3 eV (black diamonds), and fit (solid red line). The dashed gray line indicates the background signal intensity used in the fit, corresponding to the featureless defects emission in multiwalled boron nitride nanotubes (Ref. [14]). The $D_{2}$ and $D_{6}$ lines are attributed to the boron-nitride divacancy $(V_{B N})$ . Right inset: energy shift $E_{n}$ divided by the replica index $n$ versus $n$ . Left inset: zoom of the PL spectrum around 5.8 eV. (b) Emission spectrum of the $X_{LO (T) / TO (T)}$ line at 5.76 eV (black diamonds), and three replicas used in the fit in (a), obtained by a rigid energy shift of $E_{1}$ (blue diamonds), $E_{2}$ (green diamonds), and $E_{3}$ (red diamonds).
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Figure 2
Schematic representations, in the single-particle picture of the electronic band structure, of the phonon-assisted recombination processes of the $X_{LO (T) / TO (T)}$ line at 5.76 eV (left part), and of the ${\tilde{X}}_{LO (T) / TO (T) + 2 TO (K)}$ line at 5.47 eV (right part). Inset: top view of the first Brillouin zone with high-symmetry points. The green arrows correspond to the $LO (T)$ or $TO (T)$ phonons in the middle of the Brillouin zone, and the red arrows to the $TO (K)$ phonons.
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Figure 3
Normalized photoluminescence (PL) signal intensity in bulk hBN for an excitation at 6.3 eV as a function of temperature, from 10 to 130 K (a) and from 130 to 290 K (b). (c) PL intensity versus 1000/T for three different emission bands indicated by vertical dashed lines in (a) and (b). Solid lines are Arrhenius fits with an activation energy $E_{a}$ of 50 meV for the lines at 5.76 eV $(X_{LO (T) / TO (T)})$ and 5.47 eV $({\tilde{X}}_{LO (T) / TO (T) + 2 TO (K)})$ , and of 30 meV for the $D_{2} : V_{BN}$ line at 5.56 eV, possibly related to boron-nitride divacancy $(V_{BN})$ .
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Figure 4
Time-resolved photoluminescence (PL) in bulk hBN at 10 K, recorded at the energy of the $X_{LA (T) / TA (T)}$ and $X_{LO (T) / TO (T)}$ phonon replicas (left inset, Fig. 1) with identical time traces reflecting the decay of the indirect exciton reservoir (the acquisition time was three times larger for $X_{LA (T) / TA (T)}$ ), together with the system response function (dashed line).
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Figure 5
Normalized photoluminescence (PL) signal intensity on a semilog scale (gray line, left axis) as a function of detection energy for an excitation at 6.3 eV at 10 K, and PL decay time $τ_{1 / e}$ on a semilogarithmic scale (symbols, right axis) as a function of detection energy for 10 K (red squares), 100 K (green diamonds), 200 K (blue circles), and 300 K (black stars). The red (blue) shaded area of the PL spectrum indicates the spectral domain of real (virtual)-phonon assisted transitions.
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